\documentclass{standalone}
\usepackage{tikz}
\usepackage{ctex,siunitx}
\setCJKmainfont{Noto Serif CJK SC}
\usepackage{tkz-euclide}
\usepackage{amsmath}
\usetikzlibrary{patterns, calc}
\usetikzlibrary {decorations.pathmorphing, decorations.pathreplacing, decorations.shapes,}
\begin{document}
\small
\begin{tikzpicture}[>=stealth,scale=1.8]
  \draw[very thick](-1.8,-1)rectangle(1.8,1)node[below left]{按手指处};
  \draw[thin,densely dashed](-1.8,0)--(-0.1,0)node[at start,above right]{沿此线折成直角};
  \draw[thin,densely dashed](1.8,0)--(0.1,0);
  \draw(-0.1,0)--(-0.1,0.2)--(0.1,0.2)node[midway,above]{$A$}--(0.1,0)[rounded corners=4mm]--(0.5,0)--(0.5,-1)--(-0.5,-1)--(-0.5,0)[sharp corners]--cycle;
  \draw[thin,<->](-0.5,-0.5)--(0.5,-0.5)node[midway,above]{$R$};
\end{tikzpicture}
\end{document}